Adding appenders graylog2-server and graylog2-web-interface to lgo4j.xml

https://github.com/corey-hammerton/graylog2-rpms/issues/2

In prepration for using the configuration file, adding individual appenders named
graylog2-server and graylog2-web-interface to log to /var/log/graylog2/. The
graylog2-server appender logs to ${dir}/server.log and the graylog2-web-interface
appender logs to ${dir}/web-interface.log.

Each appender is a DailyRollingFileAppender to rotate their respective log file
on a daily basis.